Process for controlling a cylinder-type silk screen printing machine

ABSTRACT

A cylinder-type silk screen printing machine includes a printing cylinder that collects an object to receive the print and a screen carriage with printing blades applied against the screen pattern, which travels over the cylinder, wherein the screen pattern and cylinder are moved during the printing process at a synchronized speed. In order to reduce the consumption of printing material and hence the environmental pollution, and at the same time to ensure synchronization of the printing cylinder and screen carriage by simple technical means, the printing cylinder and screen carriage are driven independently of one another; data for adjustment of the printing travel and printing speed are programmed into an electronic control system, stored therein and processed to obtain control signals for the drives, and the relevant movement positions of the printing cylinders and screen carriage are determined during the printing process by travel measurement systems and further processed in the electronic system in order to adjust the control signals.

FIELD OF THE INVENTION

The invention relates to a method for controlling a cylinder screenprinting machine, where a printing cylinder, which takes up the materialto be printed and is driven by a drive motor, and a screen carriage,which runs above it with a squeegee lowered onto the screen stencil, aremoved at a synchronised speed during the printing cycle. The inventionalso relates to a cylinder screen printing machine with a printingcylinder which takes up the material to be printed and is driven by adrive motor, a screen carriage running above it, a squeegee which can belowered onto the screen stencil and a device for synchronising themovement of the printing cylinder and the screen frame during theprinting cycle, which operates according to the method described above.

BACKGROUND OF THE INVENTION

Two methods of this kind are known from the prior art, in which theprinting cylinder and the screen carriage are moved synchronously by asingle drive motor via a mechanical link. In both cases, the mechanicallink is made via a gear wheel positioned on the end face of the printingcylinder which engages a toothed rack positioned on the correspondingside of the screen carriage. The printing path covered by the screencarriage corresponds exactly to one complete turn of the printingcylinder.

The first method employs a so-called swing cylinder, which has a singlegear wheel on each end face which permanently engages the toothed rackmounted on the corresponding side of the screen carriage. Following theprinting cycle, the cylinder returns to the starting position, where ittakes up the next item to be printed, and the printing cycle startsagain immediately from the beginning.

The disadvantage of this method is that the swing cylinder switchesdirectly from one sense of rotation to the other without stopping, sothat the newly fed item to be printed must be grasped instantly by theprinting cylinder. In this context, it can happen that the item to beprinted is not fed onto the printing cylinder correctly or optimally,thus impairing the printing quality.

The second known method attempts to avoid this disadvantage bycontrolling the synchronisation between the printing cylinder and thescreen carriage during the forward and return passes via two gear wheelson each end face of the printing cylinder. During the printing cycle(forward pass), the first gear wheel engages the toothed rack positionedon the corresponding side of the screen carriage. At the end of theprinting cycle, this gear wheel and the toothed rack are disengaged dueto the fact that the teeth at the appropriate point on the gear wheelare recessed by milling. The toothed rack can return freely over thestopped gear wheel. The return pass of the screen carriage is achievedby a second gear wheel which engages the toothed rack mounted on thecorresponding side of the screen carriage during the return pass. Duringthe return pass, the first gear wheel is held still by a mechanicaldevice. After the return pass has been completed, the first gear wheelstarts rotating again and engages the toothed rack of the screencarriage, at which point a new printing cycle begins. The two gearwheels are driven by a single drive motor. Due to the fact that theprinting cylinder linked to the first gear wheel always rotates in thesame direction and stops during the return pass of the screen carriage,there is sufficient time to feed the material to be printed onto theprinting cylinder, given an appropriate printing speed.

However, the disadvantage of the second method is the technicallycomplex mechanical synchronisation of the two gear wheels. This alsoleads to correspondingly high susceptibility to failure and torelatively frequent servicing of the cylinder screen printing machine.In addition, the printing speed is limited by the mechanical elements.Every increase in the printing speed also leads to a decrease in theidle time of the printing cylinder during the return pass of the screencarriage, which impairs the accurate feeding of the material to beprinted onto the printing cylinder. On the whole, the economicefficiency of the two methods is very unsatisfactory due to thedisadvantages described.

SUMMARY OF THE INVENTION

The present invention is based on the task of developing a method forcontrolling a cylinder screen printing machine which is characterised byhigh printing quality, great economic efficiency, simple technicalequipment and high printing speeds. It is also the task of the presentinvention to develop a corresponding cylinder screen printing machinewith the advantages described above.

As a solution to the task concerning the method, it is proposed inaccordance with the invention that the printing cylinder and the screencarriage be driven independently of one another and that, during theprinting cycle, the drive motor of the screen carriage be disengagedfrom the movement of the screen carriage which is synchronised with theprinting cylinder and that the drive motor subsequently drive the screencarriage during the return pass, the printing cylinder being brought toa standstill before the return pass is completed.

Based on this solution in accordance with the invention, it is possibleto accurately feed the material to be printed onto the printing cylinderduring a sufficient idle time of the printing cylinder, which can amountto the entire return time of the screen carriage, or at least theremaining portion thereof, even at a high printing speed. The technicaleffort involved in implementing the method is relatively small. Althoughtwo drive motors are used to drive the printing cylinder and the screencarriage, the method can be realised with considerably simpler technicalmeans than the second method described at the beginning and is alsocharacterised by a less susceptibility to failure and easier servicing.

A preferred improvement of the method envisages that the drive motor ofthe screen carriage be brought to a standstill during the printing cycleand the drive motor of the printing cylinder before the return pass ofthe screen carriage has been completed. The drive motor of the printingcylinder, which comes to a standstill before the return pass has ended,holds the cylinder in the position in which the material to be printedis fed on during this time.

As a second solution to the task described above, it is proposed inaccordance with the invention that the printing cylinder and the screencarriage be driven independently of one another, data for setting theprinting path and the printing speed be programmed and stored in anelectronic controller and converted into control signals for the drives,and that the respective movement positions of the printing cylinder andthe screen carriage be determined by position sensor systems during theprinting cycle and further processed by the electronic controller inorder to regulate the control signals.

The advantages described above are also attained with this method inaccordance with the invention. It is also possible to adapt the printingpath and the printing speed to the surface to be printed and the otherrequirements of the respective material to be printed. In theconventional methods, the printing path covered by the screen carriageduring the printing cycle corresponds to one complete turn of theprinting cylinder and is matched to the maximum format size of thematerial to be printed. Thus, the printing ink is distributed by theflood coater over the entire screen surface, corresponding to themaximum printing format. The evaporation of the solvents contained inthe printing ink during the spreading of the printing ink on the screensurface causes environmental pollution. The consistency of the printingink also changes during spreading on the screen surface, due toevaporation and other influences, so that it must be frequently remixedin order to ensure a uniform printing quality. Due to the fact that thesqueegee travels over the maximum printing format during every printingcycle, it is also exposed to a corresponding strain and thecorresponding wear. These disadvantages are avoided by the secondsolution proposal in which the printing path can be adjusted to therespective surface of the material to be printed.

In a further development of this solution, the squeegee and the floodcoater are also activated as a function of the movement position of thescreen carriage. This enables more accurate and more simple control ofthe activation point of the squeegees, independently of the printingspeed.

On the whole, the method in accordance with the invention offers thegreatest possible flexibility as regards the printing cycle.

In accordance with the first solution, a cylinder screen printingmachine of the type described at the beginning is designed to use twoindependent drive motors to drive the printing cylinder during theprinting cycle and to drive the screen carriage during the return pass,a device with which the drive motor of the screen carriage can bedisengaged during the printing cycle (forward pass of the screencarriage) from the movement of the screen carriage, which issynchronised to that of the printing cylinder, and a device with whichthe printing cylinder can be brought to a standstill before the returnpass of the screen carriage has been completed.

In a preferred improvement, the drive motor of the screen carriage islinked to the screen carriage via a freewheel which disengages the drivemotor from the screen carriage in the sense of rotation of the motorshaft corresponding to the forward pass of the screen carriage duringthe printing cycle, and the drive motor of the printing cylinder islinked to the printing cylinder via a freewheel which disengages thedrive motor from the printing cylinder in the sense of rotation of themotor shaft corresponding to the return pass of the screen carriage.

The first freewheel can, for example, be located on the hub of thescreen carriage drive motor and be designed as an industrial freewheel.The second freewheel can be respectively located on the two gear wheelsof the printing cylinder, particularly when a gear wheel which ismounted on each end face of the printing cylinder and engages a toothedrack on the corresponding side of the screen carriage is used as adevice for synchronising the movement of the printing cylinder and thescreen carriage.

In accordance with the second proposed solution, a screen printingmachine of the type described at the beginning is designed to have twoindependent drive motors for the printing cylinder and the screencarriage, and an electronic controller, consisting of a data input unitfor setting the printing path and the printing speed, a connected PLCprocessor to convert the data into control signals for the drive motors,and two regulators respectively dedicated to the drive motors, connectedto position sensor systems for the printing cylinder and the screencarriage.

In a preferred version, the position sensor systems are designed asrotary position transducers, which are mounted on the printing cylinderand on a drive wheel for the screen carriage.

In another preferred version, glass scales can also be used instead ofrotary position transducers. Any other suitable position sensor systemcan also be considered.

BRIEF DRAWING DESCRIPTION

A preferred practical example of the invention is described in moredetail below based on the drawings. The drawings show the following:

FIG. 1 A schematic diagram of a cylinder screen printing machine and

FIG. 2 A schematic diagram of an electronic controller for the cylinderscreen printing machine illustrated in FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT

As FIG. 1 shows, the cylinder screen printing machine illustrated thereessentially consists of a printing cylinder 1 which takes up thematerial to be printed, a screen carriage 2 running above it, a squeegee3 which can be lowered onto the screen stencil and which is in raisedposition during the return pass shown in FIG. 1, and a flood coater 4which is lowered during the return pass.

In the practical example shown, the printing cylinder 1 is connected toa drive 7 via a coaxial pulley 5, connected to the printing cylinder,and a toothed belt 6. The pulley 5 has a coaxial rotary positiontransducer, which serves as a position sensor system 8 for the angulardistance covered by the cylinder from its starting position.

The screen carriage 2, which is moved in shuttle fashion by a toothedbelt 9, runs over the printing cylinder 1. The reversing movement of thetoothed belt 9 is driven by a drive wheel 10 and runs over pulley 11.The drive wheel 10 has a coaxial rotary position transducer which servesas a position sensor system 12 for the path covered by the screencarriage 2 from its starting position.

As the schematic diagram in FIG. 2 shows, the electronic controller forthe cylinder screen printing machine illustrated in FIG. 1 essentiallyconsists of a PLC processor 13 with an input unit 14 and one regulator15 and 16 each for the drive motor 17 of the printing cylinder 1 and thedrive motor 18 of the screen carriage 2.

The two drive motors 17 and 18 are controlled as follows:

Data for the printing path and the printing speed are entered via theinput unit 14--a keyboard, for example--into the processor 13 and storedthere. The processor 13 converts the data into control signals for thedrive motors 17 and 18. The control signals are channelled to thecorresponding regulators 15 and 16, where they are converted into thecorresponding currents and voltages for the drive motors 17 and 18.

The printing cylinder 1 and the screen carriage 2 are at rest in thestarting position of a printing cycle. The squeegee 3 and the floodcoater 4 are in raised position. In this position, the material to beprinted (not shown in the drawing) is fed onto the printing cylinder 1.The squeegee 3 is lowered onto the screen stencil (not shown in thedrawing) of the screen carriage 2 at the beginning of the printingcycle. The printing cylinder 1 and the screen carriage 2 begin runningat a synchronised speed resulting from the corresponding controlsignals.

The two position sensor systems 8 and 12 transmit the respectivemovement positions of the printing cylinder 1 and the screen carriage 2to the regulators 15 and 16. The movement positions are compared in theregulators 15 and 16 with the printing path specified by the controlsignals. If the movement positions correspond to the associated controlsignals, a signal is sent from the respective regulator 15 or 16 to theprocessor 13, which then transmits a control signal to regulator 15 or16 in order to stop the printing cylinder 1 or the screen carriage 2.

The printing cycle is concluded when the screen carriage 2 stops aftercovering the printing path programmed in the processor 13 and thesqueegee 3 is lifted. During this time, the printing cylinder 1continues to run in the same rotational direction until reaching thestarting position.

When the screen carriage 2 stops at the end of the printing cycle, theflood coater 4 is lowered onto the screen stencil of the screen carriage2. During the return pass of the screen carriage 2, which immediatelyfollows the printing cycle, the screen carriage 2 is return to thestarting position and stopped by a corresponding control signal. Theflood coater 4 is raised after stopping. The printing cycle is concludedas soon as the printing cylinder 1 has reached its starting positionagain and the next item to be printed is fed onto the printingcylinder 1. The printing cycle then starts again from the beginning.

Reference Numbers

1 Printing cylinder

2 Screen carriage

3 Squeegee

4 Flood coater

5 Pulley

6 Toothed belt

7 Drive

8 Position sensor system

9 Toothed belt

10 Drive wheel

11 Pulley

12 Position sensor system

13 PLC processor

14 Input unit

15 Regulator

16 Regulator

17 Drive motor of the printing cylinder

18 Drive motor of the screen carriage

We claim:
 1. A method for controlling a cylinder screen printing machinehaving a rotatable printing cylinder for taking up printable material, areciprocable screen carriage for carrying a screen stencil in printingengagement with the printable material, a squeegee for controlling inkflow on the stencil, and means for synchronizing movement of the screencarriage and printing cylinder during forward motion in a printingcycle, said method characterized by:synchronizing forward motion of theprinting cylinder and the screen carriage during a printing cycle, andindependently driving the screen carriage in a return motion back to astarting point, while continuing forward motion of the printing cylinderuntil reaching a starting point, at which point forward motion of theprinting cylinder is stopped before the return motion of the screencarriage is completed.
 2. A method as in claim 1 in which the forwardmotion of the printing cylinder is driven by a first motor and thereturn motion of the screen carriage is driven by a second motor whereinthe second motor is brought to a stop during the printing cycle and thefirst motor is brought to a stop before the return motion of the screencarriage to the starting point has been completed.
 3. A method as inclaim 1 in which the printing cylinder and the screen carriage haveindependent drives controlled by an electronic controller, wherein datafor setting a printing path length and printing speed are programmed andstored in said controller, the controller converts the data into controlsignals for the drives, and respective movement positions of theprinting cylinder and screen carriage are sensed by position sensorsystems during the printing cycle and fed to the electronic controllerfor processing to regulate the control signals.
 4. A method as in claim3 wherein a squeegee and a flood coater are activated by said electroniccontroller based upon movement of the screen carriage.
 5. A cylinderscreen printing machine having a rotatable printing cylinder for takingup printable material, a reciprocable screen carriage for carrying ascreen stencil in printing engagement with the printable material, asqueegee for controlling ink flow on the stencil, and means forsynchronizing movement of the screen carriage and printing cylinderduring forward motion in a printing cycle, characterized by:a firstdrive motor operative to drive the printing cylinder in a forwarddirection, said synchronizing means allowing return motion of the screencarriage without reverse motion of the cylinder, a second drive motoroperative to drive the screen carriage in the reverse direction forreturn to a start position, the second motor being decoupled from thescreen carriage during synchronized forward motion in a printing cycle,said first motor stopping forward drive motion of the printing cylinderprior to completion of the return motion of the screen carriage.
 6. Acylinder screen printing machine as in claim 5 wherein saidsynchronizing means includes a freewheel that connects said second drivemotor with the screen carriage to disengage the second motor from thescreen carriage during rotation of the second motor in the forwarddirection and a second freewheel that connects the first drive motorwith the printing cylinder to disengage the first motor from theprinting cylinder for reverse rotation of the first drive motor.
 7. Acylinder screen printing machine as in claim 5 wherein said first andsecond drive motors are independently directly connected with saidprinting cylinder and screen carriage respectively and saidsynchronizing means includes an electronic controller having a datainput unit for setting the printing path and printing speed, a connectedPLC processor to convert the data into control signals for the drivemotors, and two regulators for the respective drive motors and connectedto position sensor systems for the printing cylinder and the screencarriage.
 8. A cylinder screen printing machine as in claim 7 whereinsaid position sensor systems are rotary position transducersrespectively mounted on the printing cylinder and on a drive wheel forthe screen carriage.
 9. A cylinder screen printing machine as in claim 7wherein said position sensor systems include glass scales.